This invention relates to a humidity sensing apparatus and more particularly to a humidity sensor which detects humidity by measuring the change in the electrical characteristic of an element corresponding to the ambient humidity.
The need for humidity measurement and humidity control has recently been increasing and the importance of a humidity sensor is widely recognized. There are several types of humidity sensors which detect humidity by measuring the change in the electrical characteristics of an element corresponding to the ambient humidity, including electrolytic, metallic, polymeric and ceramic humidity sensors. These humidity sensors have been extensively studied and polymeric and ceramic humidity sensors have been put to practical use. Each of the sensors measures the humidity level by measuring the change in the resistance of an element or the change in the electrostatic capacity of an element as the element absorbs or releases moisture. A resistance-change type humidity sensor detects humidity by measuring the change in the resistance of an element in accordance with the ambient humidity. A capacitance-change type humidity sensor detects humidity by measuring the change in the electrostatic capacity of an element in accordance with the ambient humidity.
An electronic thermometer which measures temperature using a resistance-change type temperature sensor is known from Japanese Patent Laid-Open No. 15134/1983. The structure of the electronic thermometer may also be used to measure humidity. However, electrolysis results and moisture sensitivity changes if current flows through the humidity sensor in only one direction for a long time period. Thus, accurate humidity measurement is not possible.
Many of the conventional resistance-change type humidity sensors have such high resistance at low humidity that it is difficult to measure low humidity with high accuracy. In order to construct a humidity sensor which is able to measure low humidity with high accuracy, high quality circuitry and a highly accurate mounting technique are required, leading to an increase in manufacturing costs.
Generally, in a resistance-change type humidity sensor, the logarithm of resistance changes linearly with respect to the change in relative humidity. If the linearity is good, a logarithm amplifier is capable of compensating the linearity. In conventional humidity sensors, however, the linearity is poor. Thus, a complicated linearity compensation circuit is required to produce a highly accurate hygrometer.
Additionally, in a resistance-change type humidity sensor, the greater the rate of change in the resistance between low humidity and high humidity, the greater the sensitivity. However, when a hygrometer is produced, it is difficult to provide the dynamic range for the measuring circuit if the rate of change in the resistance between low humidity and high humidity is too large.
In a capacitance-change type humidity sensor, the linearity of change in electrostatic capacity with respect to relative humidity is poor. Thus, a complicated linearity compensation circuit is required to produce a highly accurate hygrometer. In addition, many capacitance-change type humidity sensors have poor stability at high humidity. Consequently it is difficult to measure high humidity with high accuracy.
In conventional humidity sensors moisture sensitivity is highly dependent on the temperature and a temperature compensation circuit is required. When temperature dependence is represented by a simple function, it is not necessary to have a complicated temperature compensation circuit. However, in conventional humidity sensors dependence of moisture sensitivity on temperature is not represented by a simple function and a complicated temperature compensation circuit is required to produce a highly accurate hygrometer. Although a complicated temperature compensation circuit may be provided, complete temperature compensation is difficult in a place in which the change of temperature is large. This is due to a difference in thermal response between the humidity sensor and the temperature sensor, or difference in the location of the temperature sensor and the humidity sensor. In other words, as long as moisture sensitivity of the humidity sensor is dependent on temperature, accurate humidity measurement remains difficult.
As the prior art illustrates a highly accurate hygrometer is difficult to produce. A conventional humidity sensor is expensive to manufacture since it requires high quality circuitry and a highly accurate mounting technique. Additionally, inspection and control of the humidity sensor require a large amount of time.
Accordingly, it is desirable to provide an improved humidity sensor which eliminates these problems associated with prior art devices and accurately measures humidity.